A score of 70% or better on the exam will earn you a "PS" credit for CS 105. You should talk to your academic advisor to understand what credit that satisfies.
You may only take the CS 105 proficiency exam once and may not take the proficiency if you have taken any midterm or final exams of CS 105. Additionally, you may not earn proficiency credit for CS 105 if you have credit for a more advanced programming course (eg: CS 125, CS 126, CS 205, CS 225, etc).
Find more details and sign up for the exam here.